For many years the electronics industry has used belt type furnaces for high volume cooling applications. For an application, such as chip join, the operation is characterized by loading many parts on the belt, followed by continuous movement of the belt through the furnace's heating areas to the cooling areas. It is also very important that the furnace provides a uniform temperature across the belt so that each individual part reaches the same temperature during processing. During a typical high volume cooling application, intra-part gradients and short dwell times never becomes a problem, because the parts are small and are easily cooled. For these applications the total cooling system has evolved to a point where it is very reliable and reasonably priced.
U.S. Pat. No. 4,398,700 (Thome) discloses a continuous flow annealing furnace, with cooling chambers that employ mass-acting plenums with uniform distribution of gas within that plenum.
U.S. Pat. No. 4,627,814 (Hattori et al.) discloses a furnace that has various process chambers, each of which is individually separated. Also disclosed are fans that provide uniform cooling of the process chambers.
U. S. Pat. No. 4,792,302 (Baker et al.) discloses a heating system that provides uniform air distribution system to achieve temperature uniformity. It uses an insulating block, which has evenly spaced equal size holes drilled in it to provide the uniform distribution.
U.S. Pat. No. 4,957,432 (Rachal et al.) discloses the heating and cooling from both the ceiling and the floor of a furnace. Also disclosed is the mixing of hot and cold air or gases to provide desired temperature inside the furnace chamber.
U.S. Pat. No. 4,966,547 (Okuyama et al.) discloses a heat treatment method using a zoned tunnel furnace. The furnace has roller conveyer and each of the zones in the furnace walls are provided with electric resistance heating wires. The heaters in each zone are under programmed control, independent of the heaters in the other zones. Similarly, the roller conveyer in each zone can be driven independent of the roller conveyer in the other zones by programmable controllers.
IBM Technical Disclosure Bulletin, entitled "Gas Jet Manifold For Module Cooling In Belt-Type Chip-Joining Furnaces", Vol. 33, No. 1A, Page 308 (June 1990), discloses the cooling of chip-substrate module using cooling gas through an array of nozzles that are a part of a plenum. The plenum has a diffuser through which the cooling gas enters.
The parts or products using conventional belt type furnaces have changed over time. Some of the parts have been getting larger, and it has become increasingly difficult to do the same type of processing on the larger parts, as done by the furnaces known in the art. Because of the thermal mass or thermal weight some of the larger parts resist being cooled quickly. Another factor is that newer and different materials are being used to make these parts and these newer materials require a different cooling regime. These issues are further compounded by the fact that now closer temperature control and lower intra-part gradients are being required by the electronics industry, and this has made the conventional belt furnace only marginally acceptable.
The manufacturers of conventional belt type furnaces have made quite a few upgrades to their furnaces in response to the industrial needs. Some upgrades include providing better and more efficient gas flows. Others have provided improved zone separation. And, still others are providing better cooling in the cool down section. Most of these changes are required because the parts or products are less tolerant to thermal process irregularities and the resultant mechanical stress.
For the larger parts it was observed that when these large parts were run in conventional belt furnaces they cool around the periphery faster than in the center. This is especially true for large and poor thermally conductive parts. It was noticed that this temperature gradient was as large as 50.degree. C. or larger. This non-uniform part cooling with such a high temperature gradient induces thermal stresses throughout the part and exposes the part to potential failure.
Another concern that has also surfaced is achieving minimal dwell times. Large thermal mass, high intra-part thermal conduction and a slow constant moving belt all combine to make short uniform dwells all but impossible. When a part is placed in a conventional cooldown chamber, the edges of the part cool before the center, and the entire part does not fall back to the desired temperature in as short period of time as required which results in longer dwell times.
Another problem that arises with conventional cooling schemes is the high volume of cooling gas or media used. Conventional schemes provide cooling gases to the entire cooling chamber. Basically, a fixed volume of cooling gas is constantly provided to the entire cooling chamber. This is necessary since the position of a given part is not known throughout its travel through the furnace.
For the above-mentioned reasons, processes such as chip join and pin braze on larger products cannot always be processed within specification using the belt type conventional furnaces. And, those parts that are processed, are processed at the full tolerance of the specification.
For example, the chip join process is characterized by two main parameters. The chip join process, is a process where an integrated circuit (I.C.) chip is joined to a substrate or carrier, typically using a plurality of solder balls. First, the part, such as a chip and the substrate, must go from the melting point of the alloy (Mp), i.e., the Mp of the solder balls, to a greater temperature (e.g. over 30.degree. C. past Mp) and then back to Mp. Secondly, this raising and lowering of the temperature for the chip join process must be done in minutes. This has not been a problem for most belt furnaces, as long as the part or product or carrier is in the 50 mm by 50 mm size range. Products or substrates in the 100 mm by 100 mm size range begin to present a problem due to their large thermal mass, making it very difficult to heat to the desired temperature and then cool it to its original temperature in minutes. Furthermore, rapid cooling of these parts or carriers introduces large temperature gradients. These gradients as discussed elsewhere are as large as 50.degree. C. or larger.
The belt furnace of this invention overcomes the above-mentioned and other shortcomings of the conventional belt type furnaces.